Formula	Name	Family	Number min-max	Isotope mole fraction	State	Relevance
	$Mg$	Magnesium	element	0 - 2		constituent element	main
	$Fe^{2+}$	Iron(II) cation	element	2 - 4		constituent element	main
	$Fe^{3+}$	Iron(III) cation	element	0 - 1		constituent element	main
	$Al$	Aluminium	element	2		constituent element	main
	$Si$	Silicon	element	3		constituent element	main
	$O$	Oxygen	element	18		constituent element	main
	$H$	Hydrogen	element	6 - 8		constituent element	main

Composition comments

Fe3+ is minor or nonexistent. Common impurities: Ca, Na

Chemical functions

	Function	Number
	$(OH)^-$

Chemical bonds

	Bond	Number
	$O^--H$	1

Oxides

Formula	$MgO$	$Al_{2}O_{3}$	$FeO$	$Fe_{2}O_{3}$	$SiO_{2}$	$H_{2}O$
Mass fraction (%)

Strunz

Class

silicates (09)

Division

phyllosilicate (09.E)

Family

with mica sheets, composed of tetrahedral and octahedral nets (09.EC)

Code

09.EC.55

Dana

Major class

silicates (VIII)

Class

phyllosilicate sheets of six-membered rings (71)

Type

interlayered 1:1, 2:1, and octahedra (71.04)

Group

Chlorite group (Tri-Dioctahedral) (71.04.01)

Code

71.04.01.07

Phase type

crystalline

Crystal system

monoclinic

Crystal class

prismatic

Crystal class symbol

2/m (C2h)

Crystal spacegroup

C2/m

Refringence type

biaxial

Refringence sign

negative

Birefringence

0.0700

Refraction index $n_{\alpha}$

1.595 - 1.671

Refraction index $n_{\beta}$

1.599 - 1.684

Refraction index $n_{\gamma}$

1.599 - 1.685

Dispersion V

r < v weak

Pure color

green

Color

Green, Gray, Greenish gray, Brown, Blackish green, black

Diaphaneity

translucent to opaque

Luster

vitreous

Links

• Wikipedia
• Webmineral database
• Mindat
• Handbook of Mineralogy (Min. Soc. Am)